The present invention generally relates to the machining arts and more particularly to a safety and control system for manipulating a workpiece above the surface of a vacuum bed worktable for machining while protecting the vacuum bed from damage during the machining process.
There are three vacuum clamping concepts presently known to the field, all of which rely on a vacuum source and an enclosure (vacuum bed) upon which the clamping device is located. The "vacuum bed", which may or may not be an integral part of the computer numerically controlled (CNC) machining center, creates a negative pressure environment and transfers it, via a series of holes, to its surface. In the following documentation, the term "vacuum bed" is understood to be the above described method of achieving vacuum.
The most common method of achieving vacuum clamping is to use a spoilboard. The spoilboard is secured to the vacuum bed and a series of holes are drilled (within the boundaries of a foam gasket) through it to allow vacuum pressure to be transferred to its surface from the vacuum chamber. Once the workpiece is securely held by vacuum pressure to the spoilboard, the CNC machining center can perform a varied number of operations such as routing, cutting or drilling. As the spoilboard is made of relatively inexpensive materials, any damage to the spoilboard would be negligible compared to the cost of repairing or replacing the vacuum bed itself.
The second method of achieving vacuum clamping is to use flip pods. Presently, there are two known variations of this application: the Effner flip pod system disclosed in U.S. Pat. No. 5,222,719 and the Carter flip pod system marketed by the Carter Company.
Each flip pod system includes a spoilboard having an array of cavities machined therethrough. Depending upon the size and shape of the workpiece which is to be machined and the machining process desired, a pod is selectively placed into each cavity in either its "deactivated" position (flush with its host spoilboard) or its "activated" position (elevated and sitting upon its host spoilboard). The pod is designed to sit flush with the surface of its host spoilboard cavity, and to create a seal, thus preventing the transfer of negative atmospheric pressure to the general atmosphere, when it is in its deactivated position. Prior to machining, the machine operator manually turns the pod over in a predetermined configuration to create an elevated clamping surface. Once the workpiece is placed on the activated pods, the vacuum pump is turned on, thereby creating a vacuum clamping action between the pod and the workpiece laid on it. Machining is then commenced in such a manner as to direct the tool path of the machining center through its milling process without coming in contact with the pods themselves.
The Effner and Carter flip pod systems have several disadvantages. This process is necessarily time-intensive since each pod must be manually activated or deactivated for machining. Also, these pods require a workpiece that is nearly straight in order to achieve vacuum. If a workpiece is warped, some pods will not make contact with the under surface of the workpiece. This has the undesired effect of either reducing the clamping force because of vacuum leakage or does not draw sufficient vacuum pressure to hold the part at all. Another disadvantage of the flip pod systems is their inability to accommodate many irregular shapes or small work pieces, and because of this they exclude a substantial market share of CNC manufacturing.
The third method of achieving vacuum clamping is the "pop-up" system. An example of such a system is disclosed in U.S. Pat. No. 4,723,766 to Beeding. However, currently known "pop-up" systems such as Beeding are complex and prohibitively expensive compared to other systems.
The pop-up pod systems have the same general components and activation concepts and mechanisms. They are all placed within the vacuum bed or a vacuum container and are "activated" or "deactivated" in principally the same manner. Therefore, the following description should adequately cover all patents in this category.
The Beeding pop-up pod system is composed of a vacuum bed having an array of cavities into which a quantity of pods are placed. Each of the pods are either in one of two states. The pods can be in an "active" state in which they are raised to an elevated position above the surface of the vacuum bed. Alternatively, the pods can be in an "inactive" state in which they are lowered flush with the surface of the vacuum bed. The state of each pod is regulated by commands given through a CNC controller linked to the system.
The intent of the pop-up pod is to create an elevated working surface that transfers negative vacuum pressure from the vacuum bed to the surface of the pod. The workpiece is secured to the elevated pods by vacuum pressure during the machining process allowing the machining tool to penetrate it without damaging the surface of the vacuum bed.
To elevate a selected pod, positive air pressure is directed through a spool valve to an internal pneumatic cylinder which holds the pod against a fixed stop. Once the desired pods are elevated to their active position and the workpiece is placed on them, the machining program commences by turning on the vacuum pump (securing the material blank) and performing the desired machining operation. At the end of a machining operation or a multiple of the same operation (generally termed a "run"), all pods are retracted to their inactive position.
Though an advance in the automated machining art, vacuum bed systems constructed according to the Beeding reference include some inherent disadvantages. First, the pop-up systems constructed according to Beeding are too complex and expensive compared to conventional systems to make much of a commercial impact. With the Beeding system, the workpiece is raised only slightly above the working surface which is a highly machined surface with intricate vacuum clamping assemblies set into cavities. If a tool is misprogrammed in the vertical Z-axis, either or both the tool (along with its housing or bearings) and the workpiece is damaged or destroyed. Additionally, the Beeding pop-up system is not flexible enough to perform a variety of machine table functions such as load/unloading, clamping and the like which facilitates the machining process. Finally, Beeding by design is not capable of accommodating irregular shapes common to CNC manufacturing. For example, the Beeding pods are positionable in either a fully raised position or a fully lowered position. If the workpiece has an irregular surface, the vacuum clamping of the pods on the surface of the workpiece would be seriously impaired due to vacuum leakage.
Accordingly, the need arises for a vacuum bed system which provides a flexible, modular design in an automated bed which overcomes the complexity and expense of the prior art.